(1) Field of the Invention
The present invention relates generally to piezoelectric transducers and more specifically to a sealed acoustical element using conductive epoxy.
(2) Description of the Prior Art
Acoustic projectors and hydrophones for the underwater detection or transmission of acoustical signals typically employ radially-polarized piezoelectric transducers and hydrophones. These transducers and hydrophones generally comprise a piezoelectric cylinder with caps affixed to the ends of the cylinder. The cylinder comprises a layer of piezoelectric material located between an inner and an outer electrode. An electrical lead threaded through an opening in either the end cap or the wall of the cylinder provides the required electrical connection to the inner electrode of the transducer. The lead usually is soldered to the electrodes.
Piezoelectric transducers can either detect or transmit acoustical signals. During detection of acoustical signals, the signal being detected applies a pressure wave or other form of vibrating excitation to the exterior of the cylinder causing the cylinder to expand and contract mechanically in response to the applied excitation. The expansion and contraction of the cylinder produces a corresponding electrical charge, which is then transmitted to the electrodes of the piezoelectric cylinder. During transmission of acoustical signals, an electric signal of appropriate magnitude and frequency is sent to the electrodes of the piezoelectric cylinder and excites the piezoelectric layer, thereby causing the cylindrical walls to expand and contract radially. Expansion and contraction of the cylindrical walls then transmit vibrating waves across an acoustical medium such as water.
In underwater acoustical projectors and hydrophones, a rubber housing or "boot" filled with an insulating liquid surrounds the piezoelectric transducer and prevents sea water permeation. The insulating liquid electrically insulates the transducer and also dissipates heat. One concern arising for underwater devices is the potential for leaks which can occur around the internally connected electrical leads resulting in the development of electrical shorts and a reduction in the operational life of the device.
For example, the prior art in U.S. Pat. No. 4,782,470 by Poturnicki et al. discloses a hydrophone which comprises a pair of piezoelectric cylinders bonded end to end with epoxy. Each end of the composite cylinder has a ceramic cap attached, and one of these caps has a central openings for entry of a cable having two electrical leads. The leads are soldered to terminals located on a plug affixed to the interior of the cap.
Wood in U.S. Pat. No. 4,821,244 discloses a tubular acoustic projector for underwater use having a configuration similar to Poturnicki. The acoustic projector comprises a cylindrical piezoelectric transducer with end closures. One of the end closures includes a tube connected to a flat disk having either two or four bores. Electrical leads pass through the tube and the bores to the interior of the piezoelectric cylinder and are soldered to the side wall and the other end closure of the transducer.
Geil et al. in U.S. Pat. No. 4,933,919 discloses a hydrophone with an alternative configuration. The hydrophone comprises a pair of piezoelectric cylinders arranged end to end and having ceramic end caps. Each cylinder has a notch along its side wall, and the notches are aligned to meet in the center of the stacked cylinders so as to create an opening. In this embodiment, a pair of electrical leads pass through the opening in the side wall instead of the end cap and connect to the interior of the cylinder.
Shirley et al. in U.S. Pat. No. 4,565,645 also disclose an acoustic transducer comprising two metallic end caps affixed to a piezoelectric cylinder and an outer cylinder. The piezoelectric cylinder comprises a layer of piezoelectric material located between an inner and an outer electrode. A non-conductive bonding agent bonds the metallic end caps and the outer cylinder to the piezoelectric cylinder. In addition, the bonding agent electrically insulates the end caps and outer cylinder from the piezoelectric cylinder. Shirley et al., however, do not describe the internal electrical connection of the electrodes to either of the end caps. Instead, Shirley et al. concentrate on disclosing a mechanically rigid cylinder with high sensitivity and low resonance frequency.
All of these prior art devices require interior electrical leads within the piezoelectric cylinder which raises the flexibility and ultimately the acoustic performance of the transducer. For example, both Poturnicki and Wood require soldering the leads to the interior of the transducer. This soldering increases the risk of depolarization of the piezoelectric cylinder and alters its aging curve. The hydrophone of Geil et al., on the other hand, while not using solder, still has reduced acoustic performance because its configuration is susceptible to leaks around the entry of the electrical leads. Shirley et al., although not describing an internal electrical connection, would seemingly require an internal connection because of its use of non-conductive epoxy. Internally-connected electrical leads also increase the difficulty of fabrication and repair of the piezoelectric transducers.